teach copy_page_to_iter() to handle compound pages

	In situation when copy_page_to_iter() got a compound page the current
code would only work on systems with no CONFIG_HIGHMEM.  It *is* the majority
of real-world setups, or we would've drown in bug reports by now.  Still needs
fixing.

	Current variant works for solitary page; rename that to
__copy_page_to_iter() and turn the handling of compound pages into a loop over
subpages.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 8f5ce5b..12fb04b 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -957,11 +957,9 @@ static inline bool page_copy_sane(struct page *page, size_t offset, size_t n)
        return false;
 }
 
-size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
+static size_t __copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
                         struct iov_iter *i)
 {
-       if (unlikely(!page_copy_sane(page, offset, bytes)))
-               return 0;
        if (i->type & (ITER_BVEC | ITER_KVEC | ITER_XARRAY)) {
                void *kaddr = kmap_atomic(page);
                size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
@@ -974,6 +972,30 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
        else
                return copy_page_to_iter_pipe(page, offset, bytes, i);
 }
+
+size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
+                        struct iov_iter *i)
+{
+       size_t res = 0;
+       if (unlikely(!page_copy_sane(page, offset, bytes)))
+               return 0;
+       page += offset / PAGE_SIZE; // first subpage
+       offset %= PAGE_SIZE;
+       while (1) {
+               size_t n = __copy_page_to_iter(page, offset,
+                               min(bytes, (size_t)PAGE_SIZE - offset), i);
+               res += n;
+               bytes -= n;
+               if (!bytes || !n)
+                       break;
+               offset += n;
+               if (offset == PAGE_SIZE) {
+                       page++;
+                       offset = 0;
+               }
+       }
+       return res;
+}
 EXPORT_SYMBOL(copy_page_to_iter);
 
 size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,